1. Field of the Invention
The invention concerns a parking gear for a motor vehicle, the gear including an engagement wheel mounted on the drive shaft and a locking element which can engage the engagement wheel.
2. Description of the Related Art
Parking gears are generally known in motor vehicles with hydrodynamic torque converters and serve to prevent unintended movement of the vehicle when the engine is shut off, such that it is customary to lock the transmission output shaft, which is nonrotationally connected with the drive gears. To accomplish this, it is possible, for example, to mount a ratchet wheel nonrotationally on the transmission output shaft. The ratchet wheel can be controlled by a pawl operated by the driver of the vehicle.
There is a need for a parking gear that can already be engaged while a vehicle is still rolling to a stop at speeds starting at about 4 km/h without causing any damage. Loads on the pawl and the ratchet wheel during engagement of the emergency brake as the vehicle is coming to a stop or rolling on a slope are moderated in a conventional vehicle by its drive train. In this connection, the rotating masses that must be abruptly braked, such as the clutch disk and transmission shafts, are relatively small.
Parking gears are also used in electrically driven motor vehicles, and in this case, the transmission input shaft and the ratchet of the emergency brake are rigidly connected with the rotor of the electric motor. If the parking gear is abruptly engaged as the vehicle is coming to a stop, the residual rotational energy of the rotor of the electric motor can act fully on its drive train, which, in contrast to the drive train in a conventional vehicle, is relatively short, i.e., it can also act on the pawl and the ratchet wheel. To prevent failure of the parking gear in this case, suitable means must be employed to dissipate the residual rotational energy of the rotor.
It is well known that the pawl can be elastically mounted, as is described in DE 195 35 239 C1. A disadvantage here is that, when the ratchet wheel is suddenly locked, the finely tuned engagement geometry of the pawl and ratchet wheel is unacceptably altered. Alternatively, the ratchet wheel is connected as a torsionally weak spoked wheel with the transmission input shaft, and undesired oscillations are damped with a device similar to a disk spring. However, the spring effect requires a suitable special material, and the requisite frictional forces require a large contact force or a large frictional diameter. Furthermore, dry operation results in disadvantageous variations in the coefficients of friction.
DE 100 29 628 A1 provides for a multi-disk slip clutch pretensioned with a disk spring as a means of dissipating the residual rotational energy. In dry operation, negative changes in the coefficients of friction on the disks are also to be expected in this solution to the problem. Furthermore, in the case of radial limitation of the space available for the component parts, several frictional surfaces, i.e., several disks, are necessary, which in turn leads to an increased axial assembly space. On the other hand, limitation of the assembly space in the axial direction results in relatively large frictional diameters.
In addition, DE 195 33 141 C1 proposes a conical friction device that is pretensioned by a disk spring as a slip clutch for dissipating residual rotational energy. Here again, we run into the above-described problems with respect to limitation of assembly space and dry operation.